Just think about yourself, what would you like to see as a comment under your track?! As great as constructive comments are, are also affirming ones that may boost your confidence. Over all this time I've developed this funny habit when I feel a bit down, I just go to soundcloud and let all of the wonderful comments boost me back up again. It's a total treat to me.
Seems like one should make a point for the constructive ones, but that almost goes without saying. Especially in the beginning, when I first joined the OSC, I did get some key comments for my engineering development that massively pushed me forward!
All in all, comments are precious, whether they are praises or constructive. It does take some maturity, though, to stay on the real side, focus on the music and not on the musician or a silly vendetta. In 15 months it only happened once that someone acted childish and I screwed up a little once myself in a mildly worn down state, hahaha...woops.
...but, yeah, just think about what you'd enjoy to find under your song!